Method of preparing magnesium bisulfite cooking liquor



Oct. 30, 1951 E. HAzELQUlsT METHOD OF PREPARING MAGNESIUM BISULFITE COOKING LIQUOR vFiled March 3o, 195o Patented Oct. 30, 1.1951

UNITEDy STATES PATENT OFF ICE.

METHOD F PREPARING'(IWAGNESIUIYTA BISULFITE'` COOKINGTIIQUOR Svarre E. Hazellluist,v Longview; Wfashi, assignor to Weyer-haeuser Timber Company, Tacoma, Wash., a4 corporation of Washington.

Application March30, 1950, Serial No; 152,800`

This. invention relates; to the production of paper pulp byl theI magnesium base sulphite process, and more particularly to an improvemennin the.v regeneration of the magnesium bisulphite liquor as@ described, for; example, inthe patentto Palmrose No. 2,351,780'.

ther magnesium hase4 sulphite, process.v the waste liquor from the digesters. isseparatedfrom the pulpA and,` after concentration in an evaporator system, the waste liquor isburnecl'insuitable furnaces to recover sulphur dioxide and an. ash consistingv of magnesium oxide. The ash iszmixed with water to. form, a slurry and is fed to. towers irrwhich. itcontacts and, absorbs sulphur dioxide from.V the; furnace. gases resultingy fromv the burning: of the waste liquor, thus producing magnesium bisulphite. liquor which,v after fortification with additional sulphur dioxide from sulphur burners', is suitable for` use as. a. cooking liquor inthe digesters.

Magnesium bisulphite is very soluble, and consequently the procedure. is well adapted to produce the cooking.- liquor. I-loweverl magnesium monosulphiteis only sparingly.y soluble in water (02.62% MgSOx at C The presence of some monosulphite in the liquor is desirable; Itis necessary, however, to avoid thev formation: of magnesium monosulphiteV in excess of itssolubil'.- ity in; the` liquor. Otherwise. the. monosulphite will.v precipitate and will be filtered out. of the system` with. resulting. loss of magnesium; oxide and sulphur dioxide.

The reaction` of. the sulphur.l dioxide with! the magnesium oxide slurry. inthe towers must, therefore,; be carefully controlled to; avoid the formation of magnesium monosulphite in excess of. its solubility` in theiliquor. In` practical operation, it hasv beenfound. that the reaction proceeds without the formation of magnesium monosulphite in excess of its solubility in the liquor, provided the absorption operates. within a p Ha rangeof 4.0 to 4.5.

It has been found desirable to feed the slurry of magnesium oxide and Water at a substantially fixed. rate. The furnace gasesv from the burning of the waste liquor are very lowin-sulphurdioxide concentration, carrying approximately 1 of sulphur dioxide under vusual conditions. Moreover, thezvolume of such gases availablefis variable; over arfwide range. Hence wehavey the problem., of keeping the acidity of the reaction materials substantially constant and within the range of 4.0 to 4.5.

It is the object of the present invention to control the regeneration of magnesium bisulphite more effectively and thus to ensure continuous and efficient operation of the absorption plant.

Another object of the invention is the utilization of available sulphur1 dioxide gas which may otherwise escape and. pollute the atmosphere.

Other objects'andiadvantages ofthe invention will; be; apparent as it is better understood by reference to the. following specification and the aecompanyingdrawing, in which the figurev isv a diagrammatic4 representation of an apparatus suitable; for; the; practice of the invention.`

Referringstothe drawing, 5, E and 'l represent absorption towers of;suitablefconstructionto Withstandl the `corrosive effect of the materials employed therein. Three towers;are normally used, butmoreor lessv can be used.

The .gases from the waste liquor furnaces (not shown) including sulphur dioxide, are delivered through a; pipe. 8;. to the bottom of thetower Sand pass upwardly therethrough; countercurrent to the flow of. a slurry of: magnesium oxide. and water supplied;, th.r,ough the pipe 9v. This slurry is circulated by withdrawingl it through the pipe l0 ande a circulating. pump, Ik which; returns; the slurry through the-pipe SftoA the absorption tower 5. The gases escapingA from the top of the absorption tower v5 are delivered, through aY pipe I2 -toi .the bdttomnf; the.; absorptiontower l6,111xix/'laichthey pass upwardly in Contact with a slurryof magnesium oxideand water supplied through a pipe I.3'. The slurry is withdrawn at` the bottom of. the. towelv 6.through.a pipe, t4 anda circulating pump; L5; and returnedthrough the pipe I3 to the tower' B., The gasesescaping fromV thetopofthe tower 6 are delivered through` a pipe IS tof the bottom of.` the tower l. and passV upwardly in contact with. a'slurry of magnesiumoxide and water supplied through the pipe |'l. Theslurry is withdrawn through a. pipe i8.; and a circulating pump t9 and returned throughthepipe. I1 to the tower '1.Y Spent gases. are relieved tothe atmosphere through a. pipe Zul-from thetop ofthe tower-'1.

Thermagnesiuma oxide slurry produced. by mixing magnesium oxideI with water isA introduced through apipe=2l havingbranches- 22,V 23 vand 24, so that each:k of:` theitowersl 5, t.A and 'll' is supplied continuously: -witlr'i freshmagnesium oxid'efin the form'.v of aslurry. Additional water, when-.necessary, is introducedA through a pipe 25V into the slurrycirculating inthe-tower `l-', and a portion of the.A circulating slurry iswithdrawn through a pipe-..261 andd'eliveredthrough thepipe- |4 into the tower 6. Similarly a portion of the slurry circulating in the tower 6 is withdrawn through a pipe 2l and delivered to the pipe l0 and thence to the tower 5.

It is thus apparent thatJ the gases containing sulphur dioxide are subjected successively to contact with slurry in towers 5, 6 and 'l so that substantially all of the sulphur dioxide is absorbed and converted with the magnesium oxide into magnesium bisulphite. The system up to this point operates substantially as in the Palrnrose Patent No. 2,351,780, and the advantages of such operation are fully set forth in that patent.

However, as indicated, it has been discovered that in order to avoid the formation of magnesium monosulphite in excess of its solubility in the liquor in the towers 5, 6 and 1, it is necessary to maintain the pH of the solution within the limited range of 4.0 to 4.5. Since the magnesium oxide slurry is introduced to the towers 5, 6 and l at a substantially uniform rate, any unusual drop in the quantity of gasesV supplied from the waste liquor furnaces or in the amount of sulphur dioxide contained therein would result in an increase in the pH of the solutions in the towers 5, 6 and 1, with a consequent tendency to precipitate magnesium monosulphite. To avoid this difficulty, a supply of additional sulphur dioxide is provided and introduced to the pipe 8 through a pipe 28. A Valve 29 permits control of the amount of additional sulphur dioxide supplied so that the solutions in the absorption towers 5, 6 and l may be held readily within the desired pH range, regardless of the quantity and quality of gas supplied from the waste liquor furnaces through the pipe 8. Thus, it is possible to regulate precisely the pH conditions in the towers 5, 6 and 'l and to ensure efficient operation of the absorption plant, avoiding deposition of magnesium monosulphite in the towers.

The additional sulphur dioxide may .be supplied from any source. A convenient source is available in the Vent gas from the fortifying tower 30. The magnesium bisulphite. solution produced in the tower is withdrawn continuously from the pipe 9 through a pipe 3|, thence through a filter 32 which removes any solid material.` The liquor is delivered through a pipe 33 to the top of the tower 30. Sulphur dioxide fromv a sulphur burner, for example (not shown), is introduced through a pipe 34 to the bottom of the tower 30 and passes upwardly in contact with the descending liquor to increase the proportion of free sulphur dioxide in the liquor, making it effective as a cooking liquor in the digesters.

Some of this gas is not absorbed and passes out from the top of the tower, to which the pipe 28 may be connected. The vent gas, which otherwise would be discharged to the atmosphere, is thus utilized, and as a result two advantages are secured. First, the regulation of the pH in the towers 5, 6 and 1; and second, the utilization of the sulphur dioxide which otherwise would be wasted. from the tower 30 contains enough sulphur dioxide to properly fortify the gases from the waste liquor furnaces to the point where the formation of magnesium monosulphite in excess of It has been found that the vent gas r its solubility in the liquor is prevented by maintaining the proper pH range in the towers 5, 6 and '1, and that by manipulating the valve 29 more or less of this vent gas can be delivered to and mixed with the gases from the waste liquor furnaces so that the pH range in the towers 5,

6 and 1 can readily be maintained at the optimum. 'I'he fortified magnesium bisulphite liquor is withdrawn through a pipe 35 at the bottom of the tower 30 and delivered to storage for ultimate use as a cooking liquor.

The procedure as described is lthe result of practical operation of the absorption system in towers.

Except in the manner outlined, it is extremely diiiicult to react the sulphur dioxide with magnesium oxide without forming more or less magnesium monosulphite in excess of its solubility in the liquor. Such excess, being insoluble, causes a loss of magnesium oxide and sulphur dioxide and clogs the system, reducing its eiliciency and requiring dismantlingv of equipment and cleaning thereof at frequent intervals.

Various changes may be made in the procedure within the limits of the appended claims, without departing from the invention or sacrificing the advantages thereof.

I claim:

1. In the method of producing magnesium bisulphite liquor by absorption of sulphur dioxide from waste liquor furnace gases normally containing a concentration of sulphur dioxide of the order of 1% in a slurry of magnesium oxide and water, the improvement consisting of controlling the pH of the resulting liquor within a range of 4.0 to 4.5 by introducing regulated amounts of sulphur dioxide from an extraneous source to the waste liquor furnace gases before absorption thereof and thereby preventing the formation of insoluble magnesium monosulphite.

2. In the method of producing magnesium bisulphite liquor by absorption of sulphur dioxide from waste liquor furnace gases normally containing a concentration of sulphur dioxide of the order of 1% in a slurry of magnesium oxide and water and subsequently fortifying the liquor by absorption of sulphur dioxide, the improvement consisting of controlling the pH of the resulting liquor within a range of 4.0 to 4.5 by introducing regulated amounts of residual sulphur dioxide which is not absorbed in the fortifying operation to the waste liquor furnace gases before absorption thereof and thereby preventing the formation of insoluble magnesium monosulphite.

SVARRE E. HAZELQUIST.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

1. IN THE METHOD OF PRODUCING MAGNESIUM BISULPHITE LIQUOR BY ABSORPTION OF SULPHUR DIOXIDE FROM WASTE LIQUOR FURNACE GASES NORMALLY CONTAINING A CONCENTRATION OF SULPHUR DIOXIDE OF THE ORDER OF 1% IN A SLURRY OF MAGNESIUM OXIDE AND WATER, THE IMPROVEMENT CONSISTING OF CONTROLLING THE PH OF THE RESULTING LIQUOR WITHIN A RANGE OF 4.0 TO 4.5 BY INTRODUCING REGULATED AMOUNTS OF SULPHUR DIOXIDE FROM AN EXTRANEOUS SOURCE TO THE WASTE LIQUOR FURNACE GASES BEFORE ABSORPTION THEREOF AND THEREBY PREVENTING THE FORMATION OF INSOLUBLE MAGNESIUM MONOSULPHITE. 